In many digital radio communication systems a digital signal having a plurality of data-bits is converted into an analog signal that contains substantially the same information as the original digital signal. A noise floor after the digital-to-analog conversion is typically defined by quantization noise and digital noise. The digital noise is a difference between an ideal digital signal and the actual digital signal. The quantization noise is a difference between a corresponding ideal analog signal, i.e. a signal with unlimited bit accuracy, and the ideal digital signal. In many applications it is required that a power spectral density (Watts/Hz) of out-of-band noise that appears on a certain frequency region that is located in the neighbourhood of a frequency band of the digital-to-analog converted signal does not exceed a specified limit. For example, in a WCDMA-system (Wide band Code Division Multiple Access) out-of-transmission band noise resulting in conjunction with generation of an analog transmission (TX) signal should not be too strong at a frequency band on which a reception signal (RX) of the WCDMA-system is located in the frequency domain. As WCDMA-systems use a full-duplex data transfer scheme there are demanding requirements for the out-of-transmission band noise that is generated as a side-effect of the transmission (TX) to the frequency band of the reception signal (RX).
In a typical solution according to the prior art, the out-of-band noise is attenuated with one or more analog filters that is/are arranged to have a stop-band/stop-bands in desired frequency regions in the neighbourhood of a frequency band of a digital-to-analog converted signal. An analog filter is a feasible solution for systems in which there is an analog base-band signal. In a system of this kind an analog low-pass filter (LPF) can be located between an output terminal of a digital-to-analog converter and an input terminal of an up-modulation circuitry that is arranged to mix the analog base-band signal to a desired frequency band. In some systems, however, there is no analog base-band signal. An example of a system of this kind is a radio transmitter having a digital-to-radio frequency converter (DRFC) that is arranged to perform not only a digital-to-analog conversion but also an up-modulation. In principle, an analog band-pass filter (BPF) could be used for attenuating the out-of-band noise. An analog band-pass filter can be a feasible solution for e.g. a DSL-modem that operates on a frequency range extending up to at most 2-16 MHz depending on a line-code being used. In conjunction with a mobile phone application that operates at a gigahertz range it can be, however, a too challenging task to realize a cost effective and sufficiently small analog filter the frequency response of which could have sufficiently sharp (dB/Hz) transitions between pass-bands and stop-bands.